This application claims the benefit of Korean Patent Application No. 2000-63567 filed on Oct. 27, 2000, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a transflective liquid crystal display (LCD) device implementing a color filter having varying thickness.
2. Discussion of the Related Art
As an information-oriented society rapidly develops, display devices are accordingly developed. The display device processes and displays a great deal of information. A cathode ray tube (CRT) has served as a mainstream of the display device field. However, to meet the needs of the times, a flat panel display device having small size, light weight, and low power consumption is a subject of research.
A thin film transistor (TFT) liquid crystal display (LCD) device is an example of a flat panel display device. The TFT LCD device is very thin and provides superior color display properties. For operation, a thin film transistor serves as a switching element of the TFT LCD device. The thin film transistor of the TFT LCD device switches a pixel such that the pixel controls a transmittance of light which is incident from a back light of the TFT LCD device. An amorphous silicon layer is widely used for a silicon (active) layer of the TFT, because it can be formed on a large, but relatively cheap, glass substrate at a relatively low temperature. The above-mentioned amorphous silicon TFT (a-Si:TFT) is frequently used for thin film transistors.
In general, the LCD devices are divided into transmissive LCD devices and reflective LCD devices according to whether the display uses an included or an external light source.
A typical transmissive LCD device includes a liquid crystal panel and a back light. The liquid crystal panel includes upper and lower substrates with a liquid crystal layer interposed in between. The upper substrate includes a color filter, and the lower substrate includes thin film transistors (TFT) as switching elements. An upper polarizer is arranged on the liquid crystal panel, and a lower polarizer is arranged between the liquid crystal panel and the back light. However, since the transmissive LCD transmits at most about 7% of the incident rays of light from the back light, it is very inefficient in terms of its power consumption.
For this reason, the transmissive LCD device requires a high back light brightness, and thus electric power consumed by the back light increases. A relatively heavy battery is needed to supply sufficient power to the back light of such a device. However, the battery rapidly discharges.
Unlike a transmissive LCD device, a reflective LCD device uses an ambient rays of light incident from a natural light source or an external artificial light source. Because of its low power consumption, the reflective LCD device is widely used for an electric organizer, a personal digital assistant (PDA), or the like that needs a portable display device.
For the above-mentioned reflective LCD device, an opaque material having a reflective property is selected for a pixel electrode such that the reflective pixel electrode can reflect ambient light. As mentioned previously, in the case of the transmissive LCD device, a transparent conductive material is selected for the pixel electrode such that the incident rays from a back light can pass there-through.
The reflective LCD device, however, is useless when the weather or exterior light source is dark. Accordingly, a transflective LCD device has been developed to compensate for the reflective LCD device and the transmissive LCD device. The transflective LCD device can selectively provide the reflective or transmissive mode, depending on needs of users.
FIG. 1 is a partial cross-sectional view illustrating a transflective LCD device 50 according to a related art. For the sake of convenience, just one pixel portion of the transflective LCD device 50 is shown. The transflective LCD device 50 includes an upper plate 10 (color filter substrate), a lower plate 30 (TFT array substrate), an interposed liquid crystal layer 20 therebetween, and a back light 45 disposed below the lower plate 30.
Each of the upper and lower plates 10 and 30 includes a transparent substrate 1. For the upper plate 10, a color filter 12 is formed on the lower surface of the transparent substrate 1, and an upper transparent electrode 14 is formed on the color filter 12. The upper transparent electrode 14 serves as a common electrode. In addition, a half wave plate (HWP) 18 is formed as a retardation film on the upper surface of the transparent substrate 1, and an upper polarizer 16 is formed on the HWP 18. The HWP provides a phase difference of xe2x80x9cxcex/2xe2x80x9d such that right-circularly polarized rays incident thereon are changed to left-circularly polarized when they pass therethrough. The upper polarizer 16 serves as a filter selectively transmitting some rays of incident light. That is to say, the upper polarizer 16 has an optical polarizing axis in one direction, and only the rays having the same orientation as the direction of the optical polarizing axis can pass through the upper polarizer 16.
The HWP 18 serves to improve a viewing angle quality by compensating for phase differences occurring due to users"" various viewing angles. Alternatively, a couple of quarter wave plates, which may be respectively formed for the lower and upper plate 10 and 30, can provide the same optical effect as the HWP 18 provides. However, if the HWP 18 is used, only a single HWP 18 is employed. Therefore, the HWP 18 has advantages in cost and processing time.
Still referring to FIG. 1, an insulating layer 33 is formed on the upper surface of the transparent substrate 1 of the lower plate 30, and a lower transparent electrode 32 is formed on the insulating layer 33. A passivation layer 34 and a reflective electrode 36 are sequentially formed on the lower transparent electrode 32, and a transmitting hole 31 is formed passing through the passivation layer 34 and the reflective electrode 36. In addition, a lower polarizer 40 is formed on the lower surface of the transparent substrate 1 of the lower plate 30.
When an electric field is applied across the liquid crystal layer 20, molecules of the liquid crystal layer 20 align according to the electric field. Then, the liquid crystal layer 20 refracts rays of light passing there-through such that a desired image is displayed.
The above-explained transflective LCD device has a transmissive portion xe2x80x9ctxe2x80x9d that corresponds to a portion of the lower transparent electrode 32 exposed via the transmitting hole 31, and a reflective portion xe2x80x9crxe2x80x9d that corresponds to the reflective electrode 36. The transmissive portion xe2x80x9ctxe2x80x9d has a first cell gap xe2x80x9cd1xe2x80x9d between the common electrode 14 and the reflective electrode 36. Whereas, the reflective portion xe2x80x9crxe2x80x9d has a second cell gap xe2x80x9cd2xe2x80x9d between the common electrode 14 and the lower transparent electrode 32. The first cell gap xe2x80x9cd1xe2x80x9d is designed to be larger than the second cell gap xe2x80x9cd2xe2x80x9d such that incident rays of light have the same efficiency for the transmissive and reflective modes. Specifically, the first cell gap xe2x80x9cd1xe2x80x9d is preferably about two times as large as the second cell gap xe2x80x9cd2.xe2x80x9d
The liquid crystal layer 20 provides a phase difference to light, and the phase difference of the liquid crystal layer 20 is usually determined depending on a refractive index and a cell gap thereof. For the above-mentioned LCD device, however, the liquid crystal layer 20 exhibits the same refractive index throughout the reflective and transmissive portions. Therefore, only the cell gap is the main factor to determine any difference between the phase difference of the liquid crystal layer 20 in the reflective or transmissive portion. Specifically, if the first cell gap xe2x80x9cg1xe2x80x9d is two times as large as the second cell gap xe2x80x9cg2xe2x80x9d, the transmissive portion xe2x80x9ctxe2x80x9d and the reflective portion xe2x80x9crxe2x80x9d involve a first phase difference of xe2x80x9cxcexxe2x80x9d and a second phase difference of xe2x80x9cxcex/2xe2x80x9d, respectively.
Now, passages and phase changes of the rays of incident light are explained comparing the transmissive and reflective modes. At this point, the upper polarizer 16 and the lower polarizer 40 are assumed to have polarizing axes crossing perpendicular to each other.
In case of the reflective mode, an ambient ray xe2x80x9cL1xe2x80x9d from an external light source is incident on the upper polarizer 16, and just a first linearly polarized ray passes there-through. The first linearly polarized ray is oriented in the same direction as the direction of the polarizing axis of the upper polarizer. The first linearly polarized ray subsequently passes through the HWP 18, and changes to a second linearly polarized ray perpendicular to the first linearly polarized ray, due to the phase difference xe2x80x9cxcex/2xe2x80x9d of the HWP 18. The second linearly polarized ray subsequently passes through a first portion of the liquid crystal layer 20 having the second cell gap xe2x80x9cd2xe2x80x9d, and changes to the first linearly polarized ray due to the phase difference xe2x80x9cxcex/2xe2x80x9d of the first liquid crystal portion. Then, the reflective electrode 36 reflects the first linearly polarized ray such that the first linearly polarized ray passes through the liquid crystal layer 20 again and changes to the second linearly polarized ray again. The second linearly polarized ray subsequently passes through the HWP 18 again, and changes to the first linearly polarized ray. Since the first linearly polarized ray corresponds to the polarizing axis of the upper polarizer 16, it can pass through the upper polarizer 16 in a normally white state.
In case of the transmissive mode, an incident ray xe2x80x9cL2xe2x80x9d from the back light 45 is incident on the lower polarizer 40, and just the second linearly polarized ray that corresponds to the polarizing axis of the lower polarizer 40 passes there-through. The second linearly polarized ray subsequently passes through a second portion of the liquid crystal 20 having the first cell gap xe2x80x9cd1xe2x80x9d but still remains as the second linearly polarized ray due to the phase difference xe2x80x9cxcexxe2x80x9d of the second liquid crystal portion. Then, the second linearly polarized ray changes to the first linearly polarized ray after passing through the HWP 18, and passes through the upper polarizer 16 in the normally white state, like the reflective mode.
As explained above, because the first and second cell gaps xe2x80x9cd1xe2x80x9d and xe2x80x9cd2xe2x80x9d have different values, the transmissive mode and the reflective mode provide the same efficiency for rays of light.
In another aspect, color purity should be considered in designing the transflective LCD device. In the transfiective LCD device of FIG. 1, the reflective mode implements a better color purity than the transmissive mode. In the transmissive mode, the incident ray xe2x80x9cL2xe2x80x9d passes through the color filter 12 only once. In the reflective mode, however, the ambient ray xe2x80x9cL1xe2x80x9d passes through the color filter 12 twice. That is to say, the ray is only once colored by the color filter 12 in the transmissive mode, but the ray is twice colored by the color filter 12 in the reflective mode. Therefore, there exists a difference of color purity between the reflective mode and the transmissive mode.
To avoid the above-mentioned problem, a dual color filter having a varying thickness is conventionally adopted for the transflective LCD device. FIG. 2 shows a transflective LCD device 60 having the dual color filter 62 according to the Korea Patent No. 2000-9979.
As shown, the dual color filter layer 62 has first and second portions 62a and 62b having different thicknesses. The first portion 62a having the smaller thickness corresponds to the reflective portion xe2x80x9crxe2x80x9d, whereas the second portion 62b having the larger thickness corresponds to the transmissive portion 62b. A transparent buffer layer 64 is interposed between the color filter layer 62 and the transparent substrate 1 such that a desired thickness ratio is achieved between the first and second portions 62a and 62b. The second portion 62b is preferably two times as thick as the first portion 62a such that the transmissive portion xe2x80x9ctxe2x80x9d involves the same color purity as the reflective portion xe2x80x9crxe2x80x9d.
FIG. 3 is an expanded cross-sectional view illustrating the dual color filter 62 of FIG. 2. As shown, the dual color filter 62 is interposed between the transparent substrate 1 and the common electrode 14. For the sake of convenience in explanation, the dual color filter 62 is captioned in FIG. 3. The dual color filter 62 has a plurality of sub-filters xe2x80x9cRxe2x80x9d, xe2x80x9cGxe2x80x9d, and xe2x80x9cBxe2x80x9d, a black matrix 61 disposed between the sub-filters, and the buffer layer 64 interposed between the sub-filters and the black matrix 61. Each of the sub-filters xe2x80x9cRxe2x80x9d, xe2x80x9cGxe2x80x9d, and xe2x80x9cBxe2x80x9d is divided into the first portion 62a and the second portion 62b, which correspond to the reflective portion xe2x80x9crxe2x80x9d and the transmissive portion xe2x80x9ctxe2x80x9d, respectively. As mentioned previously, the transparent buffer layer 64 is used for setting the thickness ratio between first and second portions 62a and 62b such that the second portion 62b is preferably two times as thick as the first portion 62a. 
The above-mentioned dual color filter 62, however, has some problems as follows. The sub-filters xe2x80x9cRxe2x80x9d, xe2x80x9cGxe2x80x9d, and xe2x80x9cBxe2x80x9d are difficult to form uniformly on the transparent substrate 1 where the buffer layer 64 is already formed. To form the sub-filter xe2x80x9cRxe2x80x9d, xe2x80x9cGxe2x80x9d, or xe2x80x9cBxe2x80x9d, a resin of a viscous liquid state is deposited and baked on the substrate 1 after the buffer layer 64 is formed on the substrate 1. The resin, however, has difficulty filling the concavity between the adjacent buffer layers 64 due to its viscosity. Then, as the resin is baked, the resin shrinks in an area between the adjacent buffer layers 64. Therefore, after the baking, a concave portion xe2x80x9cNxe2x80x9d (shown as a broken line) is formed on the upper surface of the second portion 62b of the sub-filter.
For example, the first and second portion 62a and 62b may be designed to be 1 m and 2 xcexcm, respectively, in thickness such that the reflective mode and the transmissive mode have the same color purity. The second portion 62b of the sub-filter, however, may have a smaller thickness of 1.5 to 1.6 xcexcm because of the above-mentioned reason. Then, a desired effect of the different thicknesses between the first and second portions 62a and 62b is deteriorated such that a color purity difference still exists between the transmissive and reflective modes.
In addition, if the dual color filter 62 has an irregular surface, the common electrode 14 formed thereon also has an irregular surface corresponding to the dual color filter 62. Then, the liquid crystal layer 20 cannot be uniformly aligned due to the irregular common electrode 14 such that the display quality is deteriorated.
Accordingly, the present invention is directed to an LCD device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a transflective LCD device having a uniform dual color filter such that a high display quality is achieved.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
In order to achieve the above object, the preferred embodiment of the present invention provides a transflective liquid crystal display device, which includes: an upper substrate and a lower substrate opposing to each other; a lower transparent electrode formed on the lower substrate; a passivation layer formed on the lower transparent electrode; a reflective electrode formed on the passivation layer, the reflective electrode having an open hole formed passing through the passivation layer such that a portion of the lower transparent electrode is exposed; a liquid crystal layer interposed between the upper and lower substrates, the liquid crystal layer having a first cell gap that corresponds to the lower transparent electrode and a second cell gap that corresponds to the reflective electrode, wherein the first cell gap is larger than the second cell gap; a color filter layer formed on the upper substrate, the color filter layer including a first portion that corresponds to the reflective electrode, a second portion that corresponds to the open hole, and a dummy pattern disposed, wherein the second portion is thicker than the first portion, and the dummy pattern is formed into the second portion; and an upper transparent electrode formed on the color filter layer.
The second portion of the color filter layer is preferably 1.2 to 2.0 times as thick as the first portion thereof.
The reflective electrode is made of an opaque material preferably including aluminum (Al) having a high reflectivity. The passivation layer is preferably made of benzocyclobutene (BCB).
The dummy pattern preferably has the same thickness as the buffer layer, and takes at most 20% area of the second portion of the color filter layer.
In another aspect, the present invention provides a method of fabricating an upper substrate for a transflective LCD device, the method includes: forming a black matrix on a substrate; depositing and patterning a transparent insulating material on the substrate having the black matrix such that a buffer layer and a dummy pattern are formed, wherein the buffer layer covers the black matrix, and the dummy pattern is disposed between the adjacent buffer layers; repeatedly depositing and patterning a plurality of color resins on the substrate where the buffer layer and the dummy pattern are formed such that red, green, and blue color filters are formed; and forming an upper transparent electrode on substrate where the color filters are formed.
Each color filter has a first portion covering the buffer layer and a second portion covering the dummy pattern, and the second portion is preferably 1.2 to 2.0 times as thick as the first portion.
The dummy pattern preferably has the same thickness as the buffer layer and takes at most 20% area of the second portion of the color filter layer.
The black matrix is preferably a single layer made of chromium (Cr).
Alternatively, the black matrix is a double layer made of chromium (Cr) and chromium oxide (CrOx).
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.